In order to detect and evaluate an anomaly of electrical insulation, there has been performed detection and evaluation of partial discharge, which is a phenomenon occurring before occurrence of dielectric breakdown. A partial discharge test has been prescribed as an insulation test for high voltage equipment. In general, the magnitude of discharge is evaluated by charge quantity (unit: coulomb C). Conventionally, such charge quantity has been measured by measuring discharge current and converting it into charge quantity, or by connecting a charge quantity evaluation apparatus to a circuit. In an electrostatic discharge test, since the rising time of the voltage generated by an electrostatic discharge tester (ESD gun) is 1 ns or less (i.e., the rising is steep), it is difficult to electrically measure generation of discharge near the tester.
Evaluating the magnitude (charge quantity) and energy of discharge in a non-contact manner has been demanded at a site where high-voltage power equipment or electrically driven or controlled electric energy equipment are manufactured, or in the field of maintaining and operating such equipment. A technique of measuring discharge current in a non-contact manner has drawn attention in view of safety, easiness of tests, and expected expansion of application fields. A UHF method of detecting radiation electromagnetic waves of discharge (measuring radiation electromagnetic waves in the UHF band (300 MHz to 3 GHz)) has drawn attention, and establishment of a standard for the UHF method as an IEC standard is in progress.
Meanwhile, measurement of electromagnetic waves has a problem in that electrical measurement becomes difficult if an environment is bad in term of electromagnetic noise. In particular, in a lightning impulse test and an electrostatic discharge test, strong electromagnetic waves serving as noise are radiated from their power supplies, and measurement is performed in a poor environment in terms of electromagnetic noise.
Also, there has been conventionally known an apparatus of measuring the number of times of generation of partial discharge in which partially discharge is judged and detected through use a photomultiplier tube which detects light emission in a container (see Patent Document 1). Further, there has been known a failure monitoring apparatus which monitors failures of electric equipment by detecting light emitted as a result of flashover or partial discharge occurring at a high-voltage portion of the electric equipment (Patent Document 2). The apparatus disclosed in Patent Document 2 judges that light emission has occurred in a container and a failure has occurred when the intensity of the detected light converted to an electric signal exceeds a predetermined level.
However, the relation between light and discharge energy is unclear. There has been demand for a technique which not only detects the number of times of generation of discharge or light emission itself, but also evaluates the magnitude and energy of discharge through optical measurement.